As is known, there are many electrical applications wherein the value of a current flowing through an electric load needs to be regulated.
In most cases, the current through the electric load has been regulated by a power transistor which may either be of the integrated or the discrete types.
The power transistor, in turn, is driven by an integrated drive circuit commonly referred to as the high side driver.
This power transistor is usually a MOS transistor having gate, source, and drain terminals. To charge the gate terminal of this transistor, a second voltage supply, higher than that to be applied to the drain terminal, must be made available.
To produce this second voltage supply, a bootstrap capacitor is employed which can be re-charged during the conduction phase of a second power transistor, for example. This transistor is itself driven by means of an integrated drive circuit referred to as the low side driver.
However, the supply voltage to the bootstrap capacitor must be high, if the efficiency of switching circuits is to be enhanced. Thus, the MOS power transistor is driven with gate-source voltages selected to have the smallest possible switch-on resistance RDSon.
A possible construction of transistors using MOS technology is illustrated by FIG. 1.
An epitaxial well II of the N type is grown over a substrate I of the P type. Body regions III of the P type and IV of the N type are then formed to respectively provide N-channel and P-channel transistors.
For example, two regions of the N+ type are formed in the region BODY III of the P type to provide the source and drain regions of an N-channel transistor. The source region and the body region III are conventionally connected together by a common terminal HSRC.
By using a conventional process of manufacturing structures such as that shown in FIG. 1, e.g., with BCDIII technology, the operation of MOS transistors at relatively high working voltages can be ensured. In particular, for circuits employing voltage bootstrap structures, working voltages may be provided whose values equal the voltage drop across the bootstrap capacitor. However, as the bootstrap voltage is increased, a bias voltage of the epitaxial well II cannot be ensured to equal the working voltage, because the breakdown voltage of the junction created between this well II and the regions III of the P type is smaller than the voltage drop across the bootstrap capacitor.